1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly, to a liquid crystal display device of a delta type pixel arrangement.
2. Description of the Prior Art
The liquid crystal display device is remarked as a thin and low power consuming display device replacing the conventional cathode ray tube. Particularly, the so-called active matrix type liquid crystal display device which employs as a driving element a non-linear element such as a TFT (Thin Film Transistor) and a MIM (Metal Insulator Metal) is specially remarked because of the beautiful display thereof.
In the liquid crystal display device, a stripe type and a delta type (or triangular type) are mainly employed as pixel arrangements. The pixel arrangements of the both types will be explained with reference to FIGS. 9 and 10. FIG. 9 is a diagram showing the stripe type pixel arrangement and FIG. 10 is a diagram showing the delta type pixel arrangement.
First, in the stripe type pixel arrangement as shown in FIG. 9, there is a configuration in which pixels 53 of R, G and B corresponding to colors of red, green and blue in color layers applied to the opposed substrate are arranged linearly in the horizontal and vertical directions. A set of three colors of R, G and B forms a dot 54.
Next, in the delta type pixel arrangement as shown in FIG. 10, there is a configuration in which pixels 53 of R, G and B corresponding to red, green and blue on the opposed substrate are arranged linearly in the horizontal direction (row direction) in the figure the as same as those in the stripe type, but in the vertical direction (column direction) in the figure, pixels 53 are shifted by a half pitch. Therefore, dot 54 consisting of the set of three colors of R, G and B is arranged in the form of a nest for the row direction. Therefore, the delta type pixel arrangement enables to provide a beautiful display without a remarkable zigzag in inclined lines and curves and is employed widely for the use of displaying particularly a natural image and the like.
Further, in the liquid crystal display device, it is important to prepare a sufficient auxiliary capacitance for sustaining an electric field applied across the liquid crystal layer in order to realize a high contrast and beautiful display.
Auxiliary capacitance is classified into two types in accordance with circuit configurations. Referring to FIGS. 3 and 4 which show equivalent circuits of one pixel, auxiliary capacitance 39 is formed between pixel electrode 13 and pre-stage scan line 12 in a case shown in FIG. 3 (this is referred to as a gate storage type). Alternatively, auxiliary capacitance 39 is also formed between pixel electrode 13 and auxiliary capacitance electrode 15 connected electrically with the opposed electrode in another case shown in FIG. 4 (this is referred to as a common storage type). The both cases have the following features. Auxiliary capacitance 39 is formed in parallel with liquid crystal capacitance 37 in any of the types.
FIGS. 5A and 5B are a diagram for explaining a conventional gate storage type liquid crystal display device of the delta type arrangement. FIG. 5A is a diagram showing a plan view of a substrate, and FIG. 5B is a diagram showing a shape of an opening.
The operation of the liquid crystal display device will be explained with reference to FIGS. 5A and 5B. The present device obtains a display by turning on a TFT with a scan signal given to scan line 12, that is, making conduction between drain electrode 14a and source electrode 14b of TFT 14 through channel 14c disposed between the both electrodes, at the moment charging up a liquid crystal capacitance between pixel electrode 13 and an opposed electrode (not depicted) and an auxiliary capacitance formed at a part between pixel electrode 13 and pre-scan line 12 with a display signal given to signal line 11, and sustaining the charges after turning off TFT 14.
Signal line 11 is bent at right angles in order to detour pixel electrode 13, and a portion shown by detour 11a of the signal line is disposed in parallel with scan line 12. Thus, a length of a region for forming the auxiliary capacitance, that is, an adjacent region 51 between pixel electrode 13mR and pre-stage scan line 121 is extremely shortened relative to a transverse width of pixel 13mR by affection of detour 11a of the signal line. Therefore, in order to form a sufficient auxiliary capacitance at auxiliary capacitance section 12a, it is necessary to extend scan line 121 toward scan line 12m in the next stage so as to invade an opening (transparent region).
Next, a conventional liquid crystal display device of the common storage type will be explained. FIGS. 6A and 6B are diagrams for explaining the conventional common storage type liquid crystal display device of the delta type arrangement. FIG. 6A is a diagram showing a plan view of a substrate, and FIG. 6B is a diagram showing a shape of an opening.
The operation of the liquid crystal display device will be explained with reference to FIGS. 6A and 6B. The mechanism of the operation is the same as that of the gate storage type liquid crystal display device, but the common storage type liquid crystal display device includes an auxiliary capacitance section 15a formed between pixel electrode 13 and auxiliary capacitance line 15. Auxiliary capacitance line 15 is formed simultaneously with scan line 12, and is connected electrically to the opposed substrate (not depicted). In the common storage type, there is no limitation to the region for forming the auxiliary capacitance by the bending of the signal line as practiced in the gate storage type.
However, auxiliary capacitance line 15 is formed on the same layer as that of the scan line. Therefore, in order to avoid a short circuit defect, it is desirable to extend a distance between scan line 11 and auxiliary capacitance line 15 as long as possible. From this reason, auxiliary capacitance line 15n is usually formed close to the medium portion between pre-stage scan line 12m and scan line 12n as shown in FIG. 6A.
In the liquid crystal display device of which pixel arrangement forms the delta type, there are the following disadvantages with respect to each of the gate storage type and common storage type.
Namely, with respect the gate storage type, as shown in FIG. 5A, signal line 11 is bent at right angles in order to go round pixel electrode 13, and the part shown by detour 11a of the signal line is arranged in parallel to scan line 12. Therefore, in order to form a sufficient auxiliary capacitance at auxiliary capacitance section 12a, it is necessary to extend scan line 121 toward scan line 12m in the next stage so as to invade opening 52 (transparent region). As the result, there is a disadvantage that a pixel opening shape is extremely disturbed, whereby the beautiful display quality which is a feature of the delta type arrangement is deteriorated. The disturbance to the shape of opening 52 is remarkable particularly in a fine liquid crystal display device in which the width of the line is larger relatively to the size of pixel 53 and opening 52 is shortened as the result.
In addition, as signal line 11 is arranged to bend at right angles, a total wiring length increases relative to that in the case where signal line 11 is disposed linearly, and a wiring resistance increases. The increase of the wiring resistance causes a disadvantage to delay the display signal.
In the gate storage type, a part of the auxiliary capacitance is formed on the scan line which is originally an opaque section. To the contrary, in the common storage type, because the whole auxiliary capacitance is formed on the region which has original opening 52, there is a disadvantage that the opening is extremely reduced. Further, in the common storage type, auxiliary capacitance line 15 is formed on the same layer as that of the scan line, which may cause a short circuit defect. In order to avoid the short circuit defect, it is desirable to extend a distance between scan line 11 and auxiliary capacitance line 15 as long as possible. Thus, auxiliary capacitance line 15n is usually formed close to the medium portion between pre-stage scan line 12m and scan line 12n. Therefore, there is a disadvantage that pixel opening 52 is divided into upper portion and lower portion by auxiliary capacitance line 15 and the shape of pixel opening 52 is extremely disturbed similar to that shown in FIG. 5A.
In addition, auxiliary capacitance line 15 intersects signal line 11 at intersection region 15b between the auxiliary capacitance line and the signal line to form an electrostatic capacity between signal line 11 and auxiliary capacitance line 15. Thus, there is also a disadvantage that there is caused the electrostatic capacity which becomes a load for the signal line delaying the display signal.
As explained above, in the liquid crystal display device, it is required to form large and smooth openings 52 of the pixel electrode 13 and in addition, to ensure sufficient capacitance of auxiliary capacitance 12. As examples of means for meeting the requirement, JPA 5-297412 discloses a method for forming the auxiliary capacitance at a periphery of the pixel electrode and JPA 8-87025 discloses a method for increasing the opening of the pixel electrode by miniaturizing the thin film transistor by adapting a self alignment structure. But there is a disadvantage in the examples that the manufacture is difficult.
Further, for the sake of preventing the disturbance of the opening shape, it is also possible to form auxiliary capacitance line 15 with a transparent conductive material such as ITO. However, if scan line 12 is made of metal, the product cost will increase undesirably because auxiliary capacitance line 15 and scan line 12 can not be formed simultaneously. It has also been considered to make scan line 12 with the transparent conductive material including ITO and form it simultaneously with auxiliary capacitance line 15. However, this is not practical because ITO has an extremely higher resistance than metals.